feat(commits): add Conventional Commits message normalizer + lint fixes

[iamlasse]

Summary

This PR brings the Conventional Commits message normalizer to the CLI, plus related lint and hook-install fixes.

Conventional Commits normalizer (commit-normalize)

• internal/commits — pure Normalize/validate functions enforcing Conventional Commits rules:
  • Allowed types: feat, fix, docs, style, refactor, test, chore, perf, build, ci
  • Type and scope are lower-cased automatically (FEAT(UI): … → feat(ui): …); unknown/missing types are hard errors
  • The ! breaking-change marker is accepted and preserved (feat!: / feat(api)!:) per Conventional Commits 1.0.0
  • Subject must be lowercase, ≤ 72 chars, no trailing period (auto-trimmed); capitalized subject is a hard error
  • Body is hard-wrapped at 72 columns with paragraph breaks preserved; bullet-list items (- / * ) stay on their own lines; git comment lines (#) are stripped
  • Normalization is idempotent and fully unit-tested (normalizer_test.go)
• nightshift commit normalize — CLI command with positional arg, --file (for commit-msg hooks), stdin fallback, and --check validation-only mode (prints nothing on success; non-zero exit when the message is invalid or not already in canonical form)
• scripts/commit-msg.sh — git commit-msg hook that normalizes the message file in place and rejects unfixable messages; install via make install-hooks
• docs/commit-messages.md — documents the format, the command, and the hook
• Makefile — install-hooks now installs the commit-msg hook too (previously it only installed pre-commit, so the documented command didn't work)

Lint fixes (lint-fix)

Fixes the 4 issues reported by golangci-lint run (default config):

errcheck (2) — cmd/nightshift/commands/commit.go

• Unchecked return values of fmt.Fprintln(os.Stdout, normalized) in the commit-normalize RunE (lines 55 and 58). Now captured and returned as an error, consistent with the surrounding error handling.

govet inline (2) — cmd/nightshift/commands/config.go

• Replaced the deprecated alias constant reflect.Ptr with reflect.Pointer in the printStruct switch (line 373) and the isZero switch (line 411).

Review fixes (iteration 2)

Addresses the code-review findings on the initial implementation:

• commit-msg hook errexit bug — with set -euo pipefail, the NORMALIZED=$(...) assignment aborted the script before STATUS=$?, making the documented error-guidance block dead code and rejecting bad messages silently. Errexit is now disabled around the invocation, so bad messages print the expected format and exit 1. Also fixed the quoted multi-word go run fallback (it previously tried to exec a binary literally named go run …) and replaced the shared /tmp error file with mktemp + cleanup trap.
• --check was a no-op — both branches printed the normalized message. --check now validates only: silent on success, non-zero exit when the message is invalid or not already in canonical form (commit.go isCanonical).
• ! breaking-change markers were rejected while the docs claimed Conventional Commits 1.0.0 compliance — now parsed and preserved with or without a scope; scope is lower-cased like the type.
• Bullet-list bodies were merged into prose (- item one - item two) — list items now stay on their own lines (with wrapped continuation lines kept in the list block so normalization remains idempotent).
• Duplicated CLI error output — removed the manual fmt.Fprintf(os.Stderr, …) (cobra already prints Error: …) and set SilenceUsage so validation failures don't dump the usage text.
• New tests: breaking-change markers, bullet-list preservation, idempotency with lists, and CLI-level tests for input resolution and canonicality (commit_test.go).

Verification

• go build ./... → succeeds
• go vet ./... → clean
• go test ./... → all packages pass (incl. internal/commits)
• golangci-lint run → 0 issues (baseline was 4)
• Smoke-tested nightshift commit normalize via positional/stdin/--file/--check with valid, bad-type, capitalized, overlong-subject, and multi-paragraph messages — output and exit codes as documented
• Re-verified after review fixes: hook empirically tested end-to-end (bad message → guidance + exit 1 + file untouched; fixable message → rewritten in place with bullets preserved, exit 0); --check silent-zero on canonical, non-zero otherwise; golangci-lint run → 0 issues

Nightshift-Task: commit-normalize
Nightshift-Ref: https://github.com/marcus/nightshift

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